Upvote:7
To be legally covered (pun intended), just make sure that you have some piece of clothing around your scrotum and the shaft.
The social aspect is quite more complex and the acceptable degree of skimpiness will not just differ between cities, but also between the different beaches in each region. Even if male thongs are not too common on mediterranean beaches, except on strict nude beaches, where even a thong may be too much, I would doubt that anyone at all would bother if an odd guest is wearing one.